Electrically supported bicycles or electric bicycles are well known and their use is becoming more common everyday and everywhere.
As it is known, such a electric bicycles are bicycles with an electrical auxiliary motor, wherein the user drives the bicycle by exerting pedal force, and wherein the auxiliary motor exerts an auxiliary drive force that depends on the exerted pedal force so that, the greater the pedal force, the greater the auxiliary drive force exerted by the auxiliary motor. In other words, the auxiliary motor is driven in order to generate a torque proportional to the pedaling force of the biker and so to the chain force.
Therefore, in order to carry out its work the auxiliary motor needs an input signal indicative for the magnitude of the exerted pedal force, or at least indicative for the magnitude of the drive force present in the chain.
To achieve this, some proposals in the state of the art measure signals indicative for the magnitude of the drive force present in the chain. For example, in WO-01/30643 a measuring signal is obtained by measuring a deformation of the bicycle frame when a cyclist exerts a pedal force or in WO-03/073057, where the bending of the rear axle is measured.
However, said examples of the state of the art are not appropriate for bicycles with a derailleur gear system, where the cyclist may choose among several gears, i.e., which chain wheel is in use. This means that the chain is displaced in horizontal direction along the rear axle to engage with another chain wheel distorting the generated measuring signal and therefore the measure exerted by the chain.
To solve this problem it is know U.S. Pat. No. 7,814,800, which describes a method and a device for obtaining a well-usable measuring chain force signal by sensing the horizontal component of the force applied to the bicycle's frame by the wheel axle end in the longitudinal axis of the bicycle.
More precisely, U.S. Pat. No. 7,814,800 describes a method for measuring the horizontal wheel axle force by fixing a device to the wheel axle end. Said device being a flexible metal plate, firmly screwed onto the bicycle frame, and to sense the relative horizontal displacement between the frame and the wheel axle end. The relative horizontal displacement of the wheel axle end is sensed by a displacement sensor made up of a magnet, linked to the flexible part and a Hall sensor linked to the fixed part of the metal plate. As the chain force is proportional to the relative displacement, it is deducted from the electric signal delivered by this displacement sensor.
However, although this type of chain force sensor works pretty well once perfectly set up, the practice brings to light some inconstancy regarding signal response.
Specifically, the Hall sensor of U.S. Pat. No. 7,814,800 varies its output voltage in response to the magnetic field created by the magnet, which at the same time varies its position due to the forces exerted to the chain. As the magnetic field varies proportionally with a third of the distance, this inconstancy is due to unavoidable tiny changes in the relative positioning between the magnet and the hall sensor in severe operating conditions.
Additionally, as this displacement must be free of constraints, the water and dust proofing of the sensor becomes a difficult task.